Gyroscopic Flight Instruments: PHAK Chapter 7

Handbook Reference

PHAK Ch 7

7.attitude-indicator-heading-indicator-turn-coordinator. Gyroscopic Flight Instruments: Attitude Indicator, Heading Indicator, and Turn Coordinator

The attitude indicator, heading indicator, and turn coordinator are the three gyroscopic flight instruments found on a typical instrument panel. Each relies on a spinning rotor and exploits one of two fundamental gyroscopic properties — rigidity in space (the tendency of a spinning mass to remain in its plane of rotation) or precession (the resultant action of an applied force that occurs 90° later in the direction of rotation). Understanding which property each instrument uses, how it is powered, and how it can fail is essential to safe instrument and partial-panel flight.

Power Sources

Gyroscopic instruments are driven either by a vacuum (or pressure) pump or by electricity. In a typical light single, the attitude indicator and heading indicator are vacuum-driven, while the turn coordinator is electrically driven. This split is intentional: if one power source fails, the pilot still has a means of determining bank and turn information. The vacuum system uses an engine-driven pump that creates suction; filtered air is drawn through the instrument case and directed against buckets cut in the gyro rotor, spinning it to roughly 10,000–18,000 RPM. A vacuum gauge (typically 4.5–5.5 in. Hg) lets the pilot verify proper operation.

Attitude Indicator

The attitude indicator (also called the artificial horizon) gives a direct, pictorial display of the airplane's pitch and bank attitude relative to the natural horizon. It uses a gyro mounted on a horizontal axis and depends on rigidity in space. The gyro remains fixed while the aircraft (and the instrument case) pitches and rolls around it; a miniature airplane fixed to the case is then displayed against a movable horizon bar tied to the gimbal.

Key points:

The bank scale at the top is typically marked at 10°, 20°, 30°, 60°, and 90°.
The pitch scale is graduated in 5° increments.
Modern indicators are free of the older limits (±60° pitch, ±100° bank) and will not tumble.
Allow about 5 minutes after engine start for the gyro to spin up before relying on it.
Common errors include slight nose-up indications during rapid acceleration and small bank/pitch errors after a 180° turn, all of which damp out within seconds in coordinated flight.

Heading Indicator

The heading indicator (also called the directional gyro) provides a stable display of magnetic heading that is free of the lead, lag, and dip errors that plague the magnetic compass. It uses a gyro mounted on a vertical axis and again relies on rigidity in space. Because the gyro itself is not magnetic, the heading indicator must be manually set to agree with the magnetic compass — done in straight-and-level, unaccelerated flight — and rechecked roughly every 15 minutes.

The gyro is subject to precession caused by bearing friction and by the rotation of the earth, which is why the indicator drifts over time. Slaved heading indicators (HSIs in many aircraft) are corrected automatically by a remote flux gate/magnetometer and require no manual resetting.

Turn Coordinator and Turn-and-Slip Indicator

The turn coordinator and the older turn-and-slip indicator both indicate rate of turn, but they are not identical:

The turn-and-slip indicator uses a gyro whose spin axis is mounted longitudinally; it senses yaw only and displays it as a needle.
The turn coordinator's gyro is canted approximately 30° from horizontal, so it senses both roll and yaw. It displays bank initially and rate of turn once established.

Both instruments work on precession: as the airplane yaws (and rolls, in the turn coordinator), a force is applied to the spinning gyro, causing it to precess against a calibrated spring. The resulting deflection is proportional to rate of turn. A standard-rate turn is 3° per second, which completes a 360° turn in 2 minutes; the index marks ("doghouses" or L/R marks) are calibrated to that rate.

Mounted below the miniature airplane is the inclinometer — a curved glass tube containing a black ball in damping fluid. The ball indicates the quality of the turn (coordination), not its rate:

Ball centered: coordinated flight.
Ball to the inside of the turn: slipping (not enough rate of turn for the bank — add rudder into the turn or reduce bank).
Ball to the outside of the turn: skidding (too much rate of turn for the bank — relax rudder or increase bank).

A useful memory aid: "step on the ball" to coordinate.

Pre-flight and Failure Recognition

During the engine run-up and taxi, the pilot should:

Verify suction in the green arc (typically 4.5–5.5 in. Hg).
Confirm the attitude indicator settles within 5 minutes with no more than a slight bank, and the horizon bar adjusts properly.
Set the heading indicator to the magnetic compass.
Verify the turn coordinator's miniature airplane banks in the correct direction during taxi turns and that the inclinometer ball swings freely to the outside of the turn.

A failed vacuum pump is one of the most insidious failures in IMC because the attitude and heading indicators degrade slowly. Cross-checking pitch and bank against the turn coordinator, airspeed indicator, altimeter, and VSI — the partial-panel scan — is the standard mitigation.

Oral Exam Questions a DPE Might Ask

Q1What two gyroscopic principles make these instruments work, and which instrument uses which?

Rigidity in space — the gyro resists displacement — drives the attitude and heading indicators. Precession — a force applied to a spinning gyro produces a reaction 90° later in the direction of rotation — drives the turn coordinator and turn-and-slip indicator.

Q2How is a standard-rate turn defined, and what does the turn coordinator actually sense?

A standard-rate turn is 3° per second, completing 360° in two minutes. The turn coordinator's gyro is canted about 30° from horizontal, so it senses both roll and yaw, displaying initial bank and then rate of turn.

Q3If the inclinometer ball is to the inside of a turn, what's happening and how do you correct it?

The aircraft is in a slip — the rate of turn is too low for the angle of bank. Correct by adding rudder into the turn (step on the ball) or reducing bank until the ball recenters.